In recent years, as computers and networks have advanced remarkably, various contents such as text data, image data, audio data, and the like are distributed as digital data. Digital data itself is free from deterioration due to aging, and can be permanently saved in a perfect condition. However, since it is easy to form copies of data, how to protect the copyrights of the contents poses a serious problem. Therefore, the security technique for copyright protection has gained more importance.
As one of techniques for copyright protections, “digital watermarking” is known. Digital watermarking is a technique for embedding various kinds of information so that no one can recognize information (watermark information) embedded in contents such as digital image data, audio data, text data, and the like, i.e., so as not to adversely influence the quality of data in which such information is to be embedded. For example, when the name of a copyright proprietor, the ID of a purchaser, or the like is embedded as watermark information to suppress use without consent by means of illicit copies.
Digital watermarking can be applied not only to copyright protection but also to detection of any tampered position of digital data by embedding information on digital data in advance, and checking consistency of the information embedded in the digital information.
In general, as a digital watermarking method, a method that embeds watermark information in a portion of digital data where a change in data hardly influences the quality is used. Therefore, “quality compared with original digital data before watermarking”, “robustness or resiliency of digital watermark”, and “information size that can be embedded” of digitally watermarked digital data suffer a tradeoff relationship.
Note that “robustness of digital watermark” means a degree of embedded information that can be extracted even after digitally watermarked digital data has undergone various conversion/edit processes. For example, conversion/edit processes of digitally watermarked digital data include “JPEG compression”, “rotation”, “translation”, and the like of a digital image.
In order to provide the robustness against such geometrical attacks, in case of, e.g., an image, a method of simultaneously embedding an alignment signal used to detect geometrical attacks such as rotation, scaling, translation, and the like has been proposed. Such alignment signal is generally called a registration signal.
When the registration signal is embedded in an image, even when the image has suffered a geometrical attack, geometrical transformation applied to the image can be calculated by analyzing the registration signal.
Therefore, since the original geometrical state of the attacked image can be detected, an original embedded position of a digital watermark can be correctly detected. When a digital watermarking algorithm or extraction algorithm which has no robustness against geometrical attacks, and a registration signal that detects geometrical transformation are embedded in combination, digital watermarking having robustness against geometrical attacks can be implemented.
However, digital watermarking as represented by patchwork and spread spectrum, which embed information using the difference between pixel values in two regions, must accurately detect the embedded position of watermark information upon extraction. Hence, when an image has undergone geometrical transformation/edit processes such as rotation, scaling, translation, and the like, it is difficult to accurately extract watermark information.